In the manufacturing of pigtailed optical transmitters, it is desirable to use short fibers for packaging and handling purposes. The actual length of the optical fiber of a pigtailed optical transmitter is determined primarily by the minimum required length for the fusion-splicing process because the optical fiber is fusion-spliced at the end of the manufacturing process of the optical transmitter in order to match its fiber length specification. Thus, from a mechanical point of view, the minimum length of an optical fiber of a pigtailed optical transmitter that can be used in its manufacturing process is limited by the fusion-splicing process.
Relatively short optical fiber, i.e., about fifty centimeters or less, carry a high level of light traveling through the cladding, or cladding modes. The cladding modes artificially increase the output light thus producing incorrect optical power measurements. This increase in output light can be attenuated by a polymer buffer layer coated onto the cladding layer which is designed to attenuate cladding modes within the length of about one meter. One example of an optical fiber with such a polymer buffer layer is the SMF-28 optical fiber, which has become somewhat of a standard in certain communication industries.
However, cladding modes in pigtailed optical transmitters or other optical modules are particularly problematic because both the core and cladding modes for a given stub fiber length are strongly dependent upon the launch condition, or how the light is coupled to the fiber. More specifically, the core:cladding modes ratio for a given stub fiber length depends upon the launch condition. Since the launch condition can differ from one module to another, the power output of a module having a given stub fiber length cannot be correlated to the output power of that module once the fiber has been fusion spliced; in other words, the correlation between optical power with cladding modes and cladding modes free is weak.
Another problem associated with the cladding modes of short optical fibers is the fact that bending the fiber can promote the escape of light from the cladding to the polymer buffer which makes the output power sensitive to mechanical vibrations thereby adversely affecting the repeatability of the optical power measurements.
In summary, attenuation of cladding modes can be very advantageous, particularly, in short single mode fibers of pigtailed optical modules. Various attenuation techniques have been tried, with limited success.
Specifically, two techniques for filtering or attenuating cladding modes include bending the fiber around a cylindrical mandrel and external spatial filtering. In the case of bending the fiber around a mandrel, the minimum length of the fiber depends upon the wavelength and the launch conditions. Consequently, the minimum length is larger than 30 centimeters thereby limiting the effectiveness of using a cylindrical mandrel to filter the cladding modes. In the case of external spatial filtering, this technique requires the use of a pinhole in front of the optical power meter to allow only the core radiation to be detected. This technique is not practical when bare fibers are employed.
Other techniques involve stripping the cladding mode using a lossy jacket or polymer buffer coated onto the outside of the cladding or other special fiber optic techniques. One example of a polymer buffer, again, is found in the SMF-28 optical fiber. However, the length of the fiber must be at least one meter in order for the polymer buffer to be effective thereby essentially not permitting this technique to be used in optical modules having short fiber lengths. Other mode stripping techniques include use of the combination of single mode, multi mode and single mode optical fibers fusion-spliced in a series. The cladding modes are stripped as they pass through the multi-mode/single mode interfaces. Still other modes tripping techniques include the use of refraction index gel or epoxy, double cladding or depressed cladding. Resort to these types of claddings result in a fiber that is substantially more expensive than the standard SMF-28 fiber.
Therefore, there is a need for a cladding mode attenuation device and method which will affectively attenuate cladding modes in short single mode stub optical fibers, without altering the core radiation, which can then be used in pigtailed optical modules.
The disclosed embodiments have been described with diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the disclosed attenuation devices and methods or which render other details difficult to perceive may have been omitted. It will also be noted that this disclosure is not limited to the particular embodiments disclosed herein.